The US space agency's Mars rovers may work for up to 240 days on the Red Planet, about 150 more than the mission team had originally projected.

Mission engineers have analysed power data for both Spirit and Opportunity which shows the vehicles are performing much better than they had expected.

It means the rovers can keep scouting Mars for many more interesting rocks.

Lead scientist Professor Steve Squyres made the announcement by satellite link-up to a Mars conference in London.

There is probably more driving in our future

Jake Matijevic, Nasa JPL

But the mission team adds that its original estimates of Mars' environment and the rovers' performance were very conservative.

The rovers use energy from the Sun to power their batteries, using triangular solar panels that sit horizontally around their waists. The panels have proven to be very efficient.

Heater issue

In addition, the rovers have not needed to use up as much power for heating because the Mars climate has been warmer than projections implied.

Spirit and Opportunity will now be able to stay longer on the surface of Mars to carry out tests, something which has taken on new importance after the discovery of firm evidence showing Mars had the wet environments to support life in the geologic past.

"They will be able to travel some distance from the landing sites to see a different type of terrain, a different type of geological formation," said Jake Matijevic, assistant engineer at Nasa's Jet Propulsion Laboratory.

"There is probably more driving in our future."

He added that the revised estimate of 240 sols, or Martian days, for the rovers' lifetime was a good estimate, but could change if the Martian environment or the health of the rovers changed.

Opportunity has been suffering power loss due to a faulty heater in its robotic arm since it landed on 25 January.

But Mr Matijevic said plans to remove the power source to the heater for part of the night would even things out between the two rovers.